1. Technical Field
The invention relates to a multipole element for focussing the corpuscular beams in conventional electron microscopes, as well as a method of producing the same.
2. Description of the Prior Art
Round lenses are used for focussing the corpuscular beams in conventional electron microscopes, electron and ion beam recording devices, ion processing and implantation devices as well as electron been measuring devices. The geometry of these lenses which produce rotationally symmetrical electric and/or magnetic fields is optimised to small aberration constants. In systems in which rotationally symmetrical electric or magnetic fields which are not dependent on time and are free of space charge are used exclusively to produce the lens effect, the spherical aberration of the third order and the axial chromatic aberration of the first order, first degree, cannot be completely corrected in principle. This aberration limits the resolution and a correction thereof is only possible if one of the restrictions regarding the fields is waived. The transition to un-round image reproducing systems, i.e. image reproducing systems which have multipole elements for beam formation, is particularly promising.
Simple multipole elements (quadrupoles) are used for example as adjustment means or stigmators in any electron microscope. For these elements the requirements regarding the shape of the field-generating components (pole pieces, electrodes), the accuracy of the adjustment to the optical axis and the long-and short-term stability are in any case substantially less than for systems for correction or reduction of aberrations (correctors), which consist for example of several strongly focussing multipole elements arranged one behind another. These must be adjusted vary accurately with respect to one another and should not be displaced relative to one another or twisted In addition the parts of the multipole elements arranged in the vacuum should have a surface which does not allow gas to escape and they should not be contaminated or charged by the primary beam.
In the electric magnetic multipole elements which are known from Optik 60, No. 3(1982) page 307 to 326, the parts arranged in the vacuum, that is to say in particular the pole pieces, the exciting windings fixed on the pole pice mounting and the supply lines, are cast integrally in synthetic resin in order to reduce the gasemitting surface. However, this embedding technique has the disadvantage that in spite of hardening for months in a vacuum the synthetic resin emits gas, and furthermore shrinks, becomes brittle and influences the magnetic properties of the multipole element. In addition, costly shielding is necessary in order to avoid charging by the corpuscular beam.